King of the Bean
(roi de la fève). The Twelfth-night king: so called because he was chosen by distributing slices of Twelfth-cake to the children present, and the child who had the slice with the bean in it was king of the company for the night. This sport was indulged in till the Reformation, even at the two universities.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894